I don't believe you can do what you want through this OQL function. You can do something like
AND DATEDIFF(YEAR, DateStarted, DateCompleted) > 1
but you cannot replace these attributes with the current datetime token. Instead, you can try setting up a date range parameter and use
AND DateStarted IN $dateRange
The users can then enter the date range to report on in the reporting form.
As another alternative, you can add a Datetime parameter and set it to a Date Constraint of Last Year